The toner and/or dry ink used in electrophotographic printing machines is a blend of materials, including plastic resins, coloring pigments and other ingredients. Most toners are produced in bulk using a melt mixing or hot compounding process. Plastic resins, carbon black, magnetic iron oxides, waxes and charge control agents are blended together while in a molten state to thereby form a hot paste. This mixture is then cooled, typically by forming it into slabs on a cooling belt or by pelletizing the mixture and cooling the pellets. The toner pellets are then ground or pulverized into a toner powder by, for example, jet mills or air-swept hammer mills. This process produces a powder having a wide range of particle sizes. The toner powder is then sifted or classified to remove over-size and under-size toner particles. Most toner powders produced today for use in electrophotographic printing processes have a volume-median particle size of from approximately 4 to approximately 14 microns.
The toner powder may then be surface treated with various additives, such as, for example, silica and charge control agents, in order to adjust various characteristics of the toner powder, such as the flow and electrostatic properties thereof. The additives are in the form of particles of a super-fine particle size, such as, for example, a volume median particle size in the sub-micron or nanometer range. Surface treatment of particulate material occurs or can be characterized along a continuum. More particularly, and with reference to FIG. 1, the degree to which a particulate material, such as a toner particle, is treated with surface treatment particles, such as silicon dioxide (SiO2), varies from a poorly dispersed condition wherein just a few surface treatment particles are clumped into adherence with the particulate material (illustrated at the far left-hand side of the continuum of FIG. 1) to an engulfed condition wherein the surface treatment particles are relatively-evenly dispersed on the outer surface of the particulate material with at least fifty-percent of their surface area being below or embedded within the surface of the particulate material (illustrated at the far right-hand side of the continuum of FIG. 1).
Conventionally, surface treatment of fine particulate material such as toner is accomplished by mechanically mixing the toner powder and additives together in a high-speed high-capacity paddle-type mixer, such as those manufactured by Hosokawa Micron Group and Henschel. Such mixers conduct a batch mixing process whereby a relatively large quantity, such as, for example, 75 to 1000 kilograms, of toner powder is surface treated. The general disadvantages of batch processes, i.e., their discontinuous and labor intensive nature, are well known. Further, the large quantity of particulate material involved in a batch process dictates that large machines be used. Large machines, in turn, take up valuable floor space within a manufacturing facility and are more difficult to clean and maintain than smaller machines. Still further, the large quantity of particulate material involved in a batch process may result in a significant amount of particulate material entering the air stream and which may undesirably form a potentially explosive cloud of particulate matter. Moreover, the conventional method of surface treating particulate material requires relatively long mixing times in order to achieve a significant degree of surface treatment. Thus, surface treatment of particulate material using conventional methods is a time consuming and relatively inefficient process.
Therefore, what is needed in the art is an improved apparatus and method for surface treating fine particulate material.
Furthermore, what is needed in the art is a continuous, rather than a batch, process for surface treating fine particulate material.
Still further, what is needed in the art is an apparatus and method for surface treating fine particulate material that processes smaller quantities of material in a continuous manner and can therefore be conducted on a smaller scale, with smaller machines that are easier to maintain and clean, and which substantially reduce the likelihood of potentially explosive conditions.
Moreover, what is needed in the art is an apparatus and method for surface treating fine particulate material that achieve a relatively high amount of surface treatment in an efficient manner.